August 28, 2009
An interesting development in recent years is the understanding that slope stability analysis can be influenced by climatic events. Most geotechnical structures are not shielded from the influences of precipitation and evaporation. However, software tools have only recently been able to fully incorporate the potential influences of climate and its influence on the slope stability failures which may partially or fully go through the unsaturated zone. There are two key elements to analyzing this type of scenario:
- Unsaturated shear strength: Any type of solution must be fully able to account for the increase in shear strength resulting from the soil being unsaturated. ( SVSlope® is the only slope stability software available with four methods of assessing unsaturated shear strength).
- Climatic coupling: Any solution must be able to handle the detailed climatic coupling on an atmospherically-exposed upper boundary. In other words, the relationship between precipitation, potential evaporation (PE), and actual evaporation (AE), must be able to be modeled.
The latest versions of SVSlope and SVFlux™ are able to model a fully combined slope stability and climatic-coupled model in a single combined numerical model. This ensures that model building and solution is a simple process and easily accomplished in a reasonable time.
The slopes around the city of Hong Kong form an excellent example of this type of analysis. An analysis of this type was performed by Prof. Del Fredlund (1993) as part of an international research effort. The analysis focused on a particular 60 degree slope which had a maximum height of 35m. A preliminary analysis of this slope using a Mohr-Coulomb type of failure mechanism yielded a factor of safety of approximately 0.9 meaning that the slope should have already failed - yet the slope was standing.
An extensive field testing program yielded that the slope was being held up by unsaturated pore-water pressures and the resulting increase in shear strength of the weathered granite. This finding raised the obvious question: "Could a specific climatic event act as a trigger to cause failure?".
The answering of this question requires a combined SVSlope / SVFlux type of analysis in which climatic events and evaporation influences on a slope can be monitored. SVFlux implements the Fredlund-Wilson-Penman approach to calculating actual evaporation from an unsaturated soil surface and can therefore accurately use climatic coupling to determine the long-term influences of climate on the soil suctions in a slope.
The climatic analysis of a slope is a little more complicated that what first meets the eye. For example, it is typical to intuitively think that the worst event would be the largest precipitation event in the last 50 years. This may not be true because storm intensitycomes into play. If there is a large precipitation event but most of it runs off then it has very small influence on the factor of safety. Long "soaker" precipitation events which last over multiple days have a higher chance of destroying suctions and adversely affecting the stability of a slope.
In the analysis presented here, an extreme rain event was applied to the slope on the first day. On the subsequent two days the evaporation was allowed to pull water back out of the slope. Pore-water pressure distributions were recorded at every 0.2 of a day and these "snapshots" were then each analyzed using SVSlope. The Fredlund equation was used in order to calculate appropriate unsaturated shear strengths.
The resulting factor of safety (FOS) vs. time can be seen in the following figure. It shows a lowering of the factor of safety immediately after the application of the precipitation events followed by a rebound of the factor of safety as the water evaporates from the soil and the shear strength of the material increases.
The next figure shows the location of the critical slip surface during the analysis. The entry and exit method of specifying the slip surface is selected. Very little movement in the critical slip surface is allowed for this analysis as the potential variation in the critical slip surface location is not the intent of this example.
Other applications of this technology include i) highway side-slopes, ii) levee design, and iii) analysis of mining structures.
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